Electronically scanned antennas such as phased arrays are replacing older-technology, mechanically scanned antennas in contexts where the higher cost of electronically scanned antennas can be justified by their superior performance. Electronically scanned antennas are also the antenna type of choice in new systems that require agility or economy of swept volume. Such antennas can be used, for example, in electronic warfare (EW) systems to increase effective radiated power (ERP) over that available from a wide-beamwidth, non-scan antenna providing the same angular coverage. System requirements may necessitate that these antennas operate over multi-octave frequency ranges and scan beams of any polarization from the broadside direction to at least 45° from broadside in any direction.
Typically, electromagnetic signals to be transmitted are supplied from the transmitter system to the antenna elements of the array via transmission lines. To maximize radiation of available power, the input impedance of the antenna elements must be matched to the impedance of the transmission lines supplying the transmission signals. For example, with dipole antenna elements, the input impedance of each dipole leg must be matched to that of its transmission line. With phased array antennas, a mutual coupling exists between elements of the array, with each element being impacted by the other elements in the array, particularly those in the immediate vicinity. The effective impedance of each element is the sum of its own impedance and the mutual impedances resulting from the surrounding elements. The antenna beam pattern of a phased array can be electronically scanned over a range of pointing angles by changing the relative phases of the signals exciting the elements in the array. However, changing the relative phases of the antenna elements alters the mutual impedance coupling among the elements; hence, the terminal impedance experienced by each element varies over a range of scan angles.
Impedance matching can be further complicated by the fact that the impedance may vary differently for different scan directions. In particular, the impedance variation from beam scanning in the plane containing the electric field vector (E-plane) of a dipole antenna element may be very different from the impedance variation from beam scanning in the plane containing the magnetic field vector (H-plane). If the impedance mismatch between the transmission line and the antenna element produces a Standing Wave Ratio (VSWR) greater than 3:1, protection circuits within each transmitter amplifier might shut down the amplifier to prevent blowout. Achieving impedance matching within this tolerance over the required broad bandwidth, polarization agility, and scan ranges can be very challenging.
Techniques have been proposed to improve impedance matching in certain types of phased array antennas. These techniques cause the impedance variation from beam scanning in the plane containing the electric field vector (E-plane) to be much more like the impedance variation from beam scanning in the plane containing the magnetic field vector (H-plane). This reduces the range of impedances that has to be matched.
More specifically, it has been shown that, with single-polarization dipole arrays, grounded metal fences that separate dipoles in their E-plane, as shown in FIG. 1, modify E-plane performance while leaving H-plane performance virtually unchanged. As shown in FIG. 1, each column of single-polarization dipole antenna elements 10 is separated from adjacent columns of antenna elements by continuous, linear metal fences 12. This phenomenon can be used to render the two scan impedances (E-plane and H-plane) close to each other, thus allowing the same matching scheme to be used for both scan directions. The mechanism at work here is that the fences provide an alternative termination for some of the dipoles' electric field that would otherwise terminate at the neighboring dipoles in the E-plane direction.
This technique, while quite effective, applies only to single-polarization dipole arrays. In dual or multi-polarized dipole arrays, the fences that separate one set of dipoles in their E-plane will separate the orthogonal set of dipoles in their H-plane, causing the magnetic field of these dipoles to be restricted. By restricting the magnetic field, these “H-plane fences” would cause more energy storage, which would reduce the bandwidth over which the array can be impedance matched. Consequently, the grounded fence scheme for improving impedance matching is not readily extendable to dual or multi-polarized antenna arrays.